The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

9784. 'In the tent of meeting' means the Lord's presence. This is clear from the consideration that the tent was made for the express purpose of providing a place for the Lord to meet Moses and Aaron, and also the children of Israel. Therefore also the holy act of worship was performed there, as the following verses in Exodus make clear,

They shall make the continual burnt offering at the door of the tent of meeting before Jehovah, where I will meet you to speak to you there. And there I will meet the children of Israel; and it shall be sanctified by My glory. And I will sanctify the tent of meeting and the altar; and Aaron and his sons I will sanctify to perform the priestly function for Me. And I will dwell in the midst of the children of Israel. Exodus 29:42-45.

And there is another place which declares that the Lord would meet them there or would be present there,

When all these things had been completed the cloud covered the tent of meeting, and the glory of Jehovah filled the dwelling-place. And Moses could not enter the tent of meeting, because the cloud dwelt over it, and the glory of Jehovah filled the dwelling-place. The cloud of Jehovah was over the dwelling-place by day, and fire was in it by night, in the eyes of all the house of Israel. Exodus 40:33-end.

From these places it becomes clear that 'the tent of meeting' means the place where the Lord's presence is. It does so because the tent represented heaven, and heaven is heaven by virtue of the Lord's presence there, on which account also it was called Jehovah's dwelling-place.